1. Field of the Invention
The present invention relates to an angular accelerometer, and relates more particularly to a thermal convection type angular accelerometer.
2. Description of the Related Art
U.S. Pat. No. 6,182,509 discloses a thermal convection type accelerometer which comprises a silicon substrate, a heater, and a pair of temperature sensitive elements. The silicon substrate has a chamber, across which the heater and the pair of temperature sensitive elements extend, and the two temperature sensitive elements are equidistant from the heater.
In order to form a heater and two temperature sensitive elements extending across a chamber, a fabrication method initially forms an oxide layer on the substrate. Next, a polysilicon layer is formed on the oxide layer. Thereafter, an oxidation process is performed to form another oxide layer on the polysilicon layer. Subsequently, the polysilicon layer is patterned to form three polysilicon bridges. Afterward, another oxidation process is performed to form oxide layers on the lateral sides of the polysilicon bridges. Next, a deep cavity is formed by etching the substrate using EDP, which is a mixture of ethylenediamine, pyrocatechol, and water.
From the above statement, it can be seen that the fabrication method for forming a heater and two temperature sensitive elements extending across a cavity is complex; thus, the thermal convection type accelerometer is expensive. Moreover, the slender polysilicon bridges are easily damaged during fabrication, resulting in low yield. Further, the slender polysilicon bridges are easily deteriorated during long term use. In addition, the substrate is of silicon, and heat from the heater can easily dissipate due to silicon having a high thermal conductivity (1.48 W/(cm-K)). Thus, the heater should be suspended over a cavity in order to consume less energy. However, when a cavity is applied, the silicon substrate still dissipates significant amounts of energy, causing such a thermal convection accelerometer to consume more energy.
Moreover, the thermal convection accelerometer can have a chamber, which is filled with carbon dioxide or air. The carbon dioxide or air may oxidize the heater and the temperature sensitive elements, resulting in poor performance and a shorter lifespan.
In addition, conventional angular accelerometers comprise piezoelectric transducers or mechanical gyroscopes. The conventional angular accelerometers are large and complex, and they can be expensive or require extended lengths of time when being repaired. With the development of MEMS technologies, small MEMS-based gyroscopes are introduced. A MEMS-based gyroscope may comprise a comb structure, and acceleration can be determined by measuring a change of parasitic capacitance or resonant frequency of the comb structure. However, the movable comb structure may be subject to mechanical fatigue. As a result, the MEMS-based gyroscope usually has a short lifespan.